Many times the user is interested in what "ad-supported small businesses" and "small publishers" (or even ad-supported big business and publishers) have to offer, so if those can't keep themselves running, there's a chance that the user will not be happy.

Besides disabling cookies doesn't mean blocking ads, so if the lack of tracking makes ads less attractive, other (possibly more intrusive) techniques must be used. I, for one, surely wouldn't want that.

That said, I think IAB has a very reasonable point there.